text:
Could a person damage a jet airliner - from the outside - with their bare hands?
Given just a small hammer, I'm pretty sure I could disable a large airliner with a few well-placed blows.
What about with my bare hands?
In other words, are there any parts of a large aeroplane that are:

exposed
delicate enough that they could be damaged without using tools
critical to the plane's operation?

The only think I can think of are the pitot tubes and angle of attack sensors.
(Throwing oneself into the engines while they are idling does not count.)

Well, there are markings on wings of where not to walk, so possibly applying your full body weight, plus jumping, on the aileron, flaps, or elevators?

People have done it by throwing coins into engines as they boarded if that counts... The engines were off at the time.

Could a strong person bend a fan blade on a non-running engine by hand?

@pericynthion  Never say never, so depending on the motor, somebody strong enough going at it long enough, might. But for most modern jet engines, it probably wouldn't be quick or easy. Fan blades are pretty tough.

@pericynthion Not unless you can bend a quarter-inch-thick metal plate with your bare hands. (Of course some martial arts experts may be able to do that...)

Jumping into a running jet engine to get sucked could be a very effective way to damage it. Even a bird can do this.

@alephzero Fan blades are mostly composite materials now.  I doubt any human could damage them by hand (with the engine not running) without some type of tool.  In normal operation they load up to hundreds of tons of force each.

So basically what I'm reading is that planes are safe havens from zombies...

What's the purpose of this question? What problem are you trying to solve?

Asking for a friend.

@PierreB Nobody told me I had to have a problem.

See https://aviation.stackexchange.com/help/dont-ask: You should only ask practical, answerable questions based on actual problems that you face. Chatty, open-ended questions diminish the usefulness of our site.

@PierreB guess she's just curious about possible options for a future experiment ;)

@PierreB "You should only ask practical, answerable questions based on actual problems that you face." The "practical ... based on actual problems you face" bit is not fit for purpose and obviously should be changed. Otherwise 26 of the top 27 (by vote) questions here (I gave up counting after that, but check https://aviation.stackexchange.com/questions?sort=votes for yourself) is ruled out.  So is every single one about aerodynamic theory, nearly every one asked by someone not in the industry, and so on.

Yes, a few possibilities...

Brake hydraulic and electrical sensor lines located on some landing gear.
Putting foreign objects under or between the rotors of brake pads
Covering pitot system sensor holes
Pressing or hitting the pitot tube (on most airliners you will need a ladder)
Letting air or hydraulic fluid out - under inflating tires or cylinders
Bending or mis-adjusting bellcranks/control arms or changing the linkage geometry.
Damaging fuel vents

I am not sure of what your definition of "exposed" is but most airliners allow the E&E (electrical & equipment) bay usually located above or behind the nose gear and gear doors to be opened and closed from external control panels for ground crews. This allows access to most the computers, batteries and thousands of wire bundles. It may also give access to lines, cables, pumps, switches, and sensors.
Opening the gear doors of some airlines exposes the air-packs, generators, converters and aux turbine with it's associated mechanisms. Maintenance and inspection panels, engine cowls and fuel distribution panels expose or control critical systems.

Just to make sure: you can do all this things with your bare hands? 5. letting them out by hand without any tool? 7. how to damage, just with bare force? and opening these panels, don't you need some tool?

Not sure some of these count. E.g. no. 2 "Putting foreign objects under...". If you have a foreign object, then is that any different to having the hammer in the OP's example?

@Mayou36 - No tools needed. Jet aircraft are different than smaller GA aircraft. Access panels, cowls, gear doors, fuel control panel do not require any tools to open. Most have levers or push buttons that unlatch with bare fingers.

@jwzumwalt Hence why physical security of the aircraft is so important!

Using foreign objects would clearly be cheating!

I expect blade antennas could be damaged with a good blow (or several) from the side. Any number of wire bundles & hydraulic lines in the wheel well could probably be damaged with enough determined yanking on them. Or disconnecting the cannon plugs - although without further damage, that by itself could be reconnected.

An interesting question is could one of the stowaways who occasionally climb into the wheel wells, either a)  damage it enough to force it to land (presumably as a self-preservation action after they discover how harsh the environment is) or b) crash the plane outright as a terror action.

Yes, it can happen.
All they'd have to do is pick up some gravel or some other pieces of FOD (Foreign Object Debris) and throw them into the engine intakes. It's probably not going to cause catastrophic damage immediately, but it will significantly increase the maintenance required to keep them operating.
I've actually heard a story about this happening once by some maintenance engineers who were upset about their overtime hours being cut, but a quick Google search isn't turning anything up - possibly because they were quietly fired after they got caught on camera and the news organizations never heard about it.


text:
Can I use actual IMC time for the requirements in 61.109(a)(3)?
I needed extra instrument time for my VFR checkride but it was cloudy for my lesson. My CFII and I turned it into an introduction to IFR lesson and we flew 0.3 actual and 0.3 simulated (with the hood) and did 1 ILS approach in IMC.
61.109(a)(3) says that the following instrument training is required:

3 hours of flight training in a single-engine airplane on the control and maneuvering of an airplane solely by reference to
  instruments, including straight and level flight, constant airspeed
  climbs and descents, turns to a heading, recovery from unusual flight
  attitudes, radio communications, and the use of navigation
  systems/facilities and radar services appropriate to instrument
  flight;

Is it correct to assume that I can use the IMC time for this?

Yes, you can. I don't have the FAR to quote, but I put 4 hours of actual instrument (along with 2 actual IFR approaches) towards my VFR PPL.

Sure can.  FAR 61.109 makes no distinction between flight in hard IMC and simulated in an aircraft using a view limiting device.

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text:
What can be considered under 61.113 (c) for passenger equipment?
Does 14 CFR 61.113(c) allow a passenger to pay for rental equipment for themselves (e.g. headset or parachute) in addition to their share of the operating expenses, or is that considered compensation?
e.g.

Occupants = 2
Hobbs cost = \$100
Headset rental for passenger = \$10

Can they pay either:

\$50 (Only allowed to split Hobbs costs)
\$55 (Allowed to include the headset in the total cost - i.e. included as 61.113 (c) rental fees)
\$60 (half the cost of the Hobbs + 100% of the headset)

Incidental costs specific to the passenger, such as O2 for the passenger's medical condition, may be paid by the passenger and need not be factored into the cost of the flight for flight sharing costs.
In your example, the Hobbs charge for the aircraft is the flight cost.  If the passenger wants a headset, or anything else that is not a flight expense, the cost need not be shared.
Landing fees, navigation fees and the like are grey areas, and I would split those.  But something like a tie-down or hangar fee is not a flight expense, and the passenger may pay for in full.
If you read cases where the FAA has prosecuted pilots for conducting operations "for hire" or similar, they tend to be egregious.
So in your example, $60 would be the right answer.  I would suggest that you could even split the cost of the charts you might buy for the trip.
As a final word, I would make certain that your passenger understands that you are just sharing expenses, not that they are "paying for a ride."

Do you have a reference for this?

What specific part of this?  I have a fat folder on this stuff because in 40 years of running a flying club it has come up numerous times, and we have had numerous speakers on the topic.

The specific part which the question asked about (incidental costs specific to the passenger).  Do you have a regulatory reference or an LOI which clearly states that the FAA allows these costs to be passed on to the passenger and not shared like the rest of the costs?  (Note that I'm not disagreeing with you, but rather than just "believing you because you said so" it would be good to have an actual reference to back up what you say.)

I will answer this issue with an additional answer to the question.

It has been asked by @Lnafziger for authority supporting an answer I have given to the question. Since this answer is not what I would call a succinct answer, I have made it separate from my initial answer.

14 CFR 61.113(c) A private pilot may not pay less than the pro rata
  share of the operating expenses of a flight with passengers, provided
  the expenses involve only fuel, oil, airport expenditures, or rental
  fees.

The regulation by context refers to operating expenses of a flight.  The OP asks about a passenger headset rental.  The regulation addresses expenses of flight.
Consider if the headset was bought by the pilot and transferred to the passenger friend, with the passenger friend reimbursing the pilot for the cost.  Would that be an expense of flight? No, it is not aircraft rental, and furthermore it is not a requirement for flight. So avoid confusing a FBO rental of a headset with a rental of an aircraft.
In the Sommer LOI by FAA Counsel, 61.113(a) is referenced, as prohibiting the "compensation or hire".  If our friend were to be given the purchased headset at the completion of the flight, that would be compensation or hire. In the LOI the FAA makes clear their long standing position that compensation or hire can include any benefit or consideration, even if there is no payment. But in the OP example, the headset was merely rented, and solely for the benefit of the passenger, and was not needed for flight as it was for personal comfort or hearing protection. Most importantly, there is no benefit to the pilot.
In the same LOI, the FAA touches upon expenses of flight not covered by 61.113(c), and while they are not enumerated, nor are examples given, the FAA makes clear that 61.113(c) applies specifically to the expenses enumerated in 61.113(c).
Sommer LOI:  https://www.faa.gov/about/office_org/headquarters_offices/agc/pol_adjudication/agc200/interpretations/data/interps/2010/sommer%20-%20%282010%29%20legal%20interpretation.pdf
In 1997 the FAA, in Harrington, made a stance that they would interpret compensation and hire broadly.  In subsequent interpretations and enforcement actions, compensation and hire were liberally construed.  In the instant case presented by the OP, even liberally construed, the rental of a headset provides no compensation to the pilot, is not necessary under regulations for the flight, and is not a fuel, oil, airport or [aircraft] rental fee.
[There are other LOI which more specifically address scenarios virtually identical to the OP's question, however, those LOIs were not found online. Sommer is more current, and touches upon the key points which need to be considered.]
As a final note, it is important when reading regulations to interpret just the regulation within the context of the body of regulation. Stated differently, avoid reading into one's interpretation issues which are separate and distinct from the body of regulation.  Furthermore, it is noteworthy that while FAA LOIs are not regulatory they provide insight as to how the FAA might deal with a matter, or might be persuaded to deal with a matter.  (Having said that, I know of cases where they appear to attempt to enforce regulations which are not codified.  Yes, the pilot can appeal to the NTSB, but a friend faced with that gave up when the FAA said at the end of highly politicized first day of the hearing that they estimated 23 days of material. His attorney advised him he had just spent $13k on the first day, and he decided to fold. An adversary is powerful armed with deep pockets.)

This is a very good answer, thumbs up!

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text:
Can paragliders in the US communicate with ATC?
I am a private pilot looking to get into paragliding. After talking to some of my friends who do paragliding in the San Francisco Bay area and watching YouTube videos I noticed some fly through places like VPMIN (which is near SUNOL). That's an extremely busy intersection, sometimes right in the altitude ranges I normally fly through them in my powered aircraft. From this I had a few questions:

Is it possible to get the equivalent of flight following or at least call up ATC to let them know you are there, if you are planning on flying at higher altitudes? I am used to doing this for a D airspace if I am flying only just over or near it if they are not busy.
If so, what would the phrasing of that sound like when you don't have a callsign?
14 CFR 103.17 says that I require prior authorization to enter class B, C or D airspace. What does this look like? Can I take a handheld radio and ask for permission on the tower frequency? Or is this something I'd need to get through written permission from a FSDO or the tower directly?

As a paraglider, you can't enter class B (or C I believe). There really shouldn't be any reason you need to talk to ATC as a paraglider in the airspaces and altitudes you'll be operating at.

You might want to look at Do lighter-than-air aircraft communicate with ATC?

Thanks a CVn that link was helpful. The one part that stuck out was this: "On the other hand, if they do have a radio (even a hand-held radio) it can greatly add to the safety of flight if they remain in contact with ATC since they can point out traffic that they might not have seen as quickly otherwise." I have always found this to be true in my powered flying and is the root of why I'm interested in doing the same for paragliding

Rob Beyer: I disagree, take this video for example https://youtu.be/vjQ6yIOAisQ the pilot here is 5500 ft right in the middle of a busy vfr corridor that is used by people who are traveling north and trying to avoid SFO B and OAK C. He is also right at a vfr cruising altitude. As far as I can tell he's perfectly legal to be there but If I was a plane flying through there I would love atc to be talking to him and so they can call out to me that he exists even if they don't know an exact location

"As a paraglider, you can't enter class B (or C I believe). "-- yes you can, if you have prior authorization from ATC, as the original asker correctly noted.

I don't think "prior authorization" would be interpretted to mean just calling up the ATC facility on the radio and asking for permission, but I may be wrong.  I'd like to see some definitive information on this.

I know of one case where paraglider pilots regularly broadcast their position via aviation-frequency radios.  They are engaged in winch towing which may involve several thousand feet of line being paid out, near an (untowered) airport.  The airspace in this location is Class G at the surface and Class E above 700'.  This is kind of an unusual case though-- normally hang glider and paraglider pilots carry radios which do not receive or transmit on the aviation frequencies.

Where noted, the information in this answer is from Advisory Circular 103-6, which was issued in 1983, but which apparently is not actually obsolete.

Is it possible to get the equivalent of flight following or at least call up ATC to let them know you are there, if you are planning on flying at higher altitudes? I am used to doing this for a D airspace if I am flying only just over or near it if they are not busy.

It's certainly legal to call up ATC while you're flying an ultralight, whether it's Part 103 or not. The relevant FCC regulation, 47 CFR § 87.18(b), says that aircraft are "licensed by rule and [do] not need an individual license," and ultralights are aircraft, so an airband radio on an ultralight counts as being licensed.
Advisory Circular 103-6 also recommends that ultralight pilots contact ATC using the ordinary radio frequencies (but I believe it was written when all airband radio users needed individual licenses).
I don't know whether or not ATC gives flight following to ultralights. Advisory Circular 103-6 says that separation services "will not be available to ultralight pilots." But there's no harm in asking.

If so, what would the phrasing of that sound like when you don't have a callsign?

There's the tricky bit.
The regulation governing this is 47 CFR § 87.107(a), which states that you should identify using one of the following four options: your "aircraft radio station call sign"; the aircraft type and registration number; your radiotelephony designator and flight number; or a temporary identifier for "organized flying activity."
However, if you're flying a Part 103 ultralight, then in all likelihood, you don't have any of those.
Advisory Circular 103-6 states that "ultralight operators should state the word 'ultralight' followed by the call letters assigned by the F.C.C. on your radio license, i.e., 'Ultralight 12593U.'" Assuming you don't have a radio license, it seems like the best you can to is to simply use the word "ultralight" as your call sign.

14 CFR 103.17 says that I require prior authorization to enter class B, C or D airspace. What does this look like? Can I take a handheld radio and ask for permission on the tower frequency? Or is this something I'd need to get through written permission from a FSDO or the tower directly?

Advisory Circular 103-6 says, "Requests for authorization to operate an ultralight vehicle into [controlled airspace] should be made by writing, telephoning, or visiting the air traffic control facility having jurisdiction over the airspace in which you wish to operate. Requests for such authorization via air traffic control radio communication frequencies will normally not be accepted, since it may interfere with the separation of aircraft."
That said, I'm not aware of any special legal meaning of the phrase "prior authorization." So on the off chance that ATC does tell you that you're allowed to enter the airspace, presumably you can do it.

Thanks a lot, this is exactly the kind of info I was looking for!

I know of some paragliding sites in the UK that are in Class D.  The site rules state that if you expect to get above a certain height you must phone the local ATC who will issue a clearance, usually 'not above xxxx feet'.  XC is not allowed from these sites.
Paraglider pilots can also create Notams or use a CANP to notify others that a class G site will be active.
Most of the radios used by paragliders and hangies in the UK are hand-held that don't cover the same bands as GA, and also aren't really usable in the air since your hands are busy with the controls.

A "push to talk" button solves the issue of hands being busy.

In the United States, it would technically not be legal to fly an ultralight aircraft (including hang gliders and paragliders) within Class D airspace (and even in some types of Class-E-to-Surface airspace) even 10' off the ground, without getting prior authorization from ATC.

This relates to Canada only
I know you’ve asked about paraglider but I shall refer to paramotor first:

You may call the ATC but  is the Navcanada that you need to let know. They are able to issue a notam. If you called the ATC you will get redirected to navcanada.
You do have a call sign. All paramotor pilots hold license and registration. Like any regular aircraft. However, unlike any regular aircraft, paramotor pilots do not have a radio. So your call sign can not be used. Funny, isn’t it?
No. You need a flight plan which automatically puts you on the map in the day of the flight.

Now regarding paragliders:

Again, you call navcanada and they will issue a notam if you are allowed to use that particular airspace. While in the air you can not be contacted directly so you have the right of way. If you participate in an xc event, the organizer does the work for you.
You do not have a call sign. You are not authorized to use aviation band.
Prior authorization means you need to obtain the right of way since you can not use radio. You will most likely not be able to obtain authorization. The authority is still navcanada.

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text:
What is the relation between clouds and radar effectiveness?
Recently, India's prime minister Narendra Modi commented on how cloudy weather can help a fighter jet  escape from radar. Is there any technical reasoning to back these comments?
Here is the quote in question:

In the interview by TV channel News Nation on Saturday, PM Modi said,
  "The weather suddenly turned bad, there were clouds... heavy rain.
  There was a doubt about whether we can go in the clouds. During a
  review (of the Balakot plan), by and large the opinion of experts was
  - what if we change the date. I had two issues in mind. One was secrecy... second, I said I am not someone who knows the science. I
  said there is so much cloud and rain. There is a benefit. I have a raw
  wisdom, the clouds can benefit us too. We can escape the radar.
  Everyone was confused. Ultimately I said there are clouds... let's
  proceed."

https://www.ndtv.com/india-news/controversy-over-pm-narendra-modis-cloud-can-help-us-escape-radar-comment-on-balakot-air-strikes-2036402

To be completely fair, that sounds like a traditional politician's way of appealing to his popular base by reinforcing his image as both a great leader whose plans are not immediately obvious ("The Lord works in mysterious ways...") and by giving the less educated voters something to feel good about ("I do not know the science but I succeeded while the know-it-alls with degrees and stuff were confused"). A rather textbook approach.

I can interpret it as (big?) stormy clouds, not only cloudy weather. What makes me wonder is: jets normally flight above those, so for who he is trying to hide

Radar relies on electromagnetic wave propagation: from a source, to the target and back to a receiver.
Anything that alters the behavior of these waves along the way will impact the performance of the radar station. For example:

A target hiding behind terrain will be harder (but not necessarily impossible) to detect, because the terrain will absorb the outgoing emissions before they even reach the target.
If the target is in plain sight but shaped in such a way that all the incoming radar energy is reflected in a direction different from that of the receiver, then it will not be detected (this is the method employed on the F-117).
If the target is in plain sight but the return signal comes back accompanied by a lot of duplicates purposefully emitted by the target, it will complicate the task of figuring out which is the correct reflection, delaying or preventing a valid solution.
If the target is coated in a material that absorbs all the incoming energy, the radar station will never see a return, and thus think there is nothing there to detect (another technique used by Low Observability aircraft). This one is relevant to your question!

The media between the emitter and the target needs to allow the waves to travel, and while the best media is vacuum, air is not terrible either. At least normal, ISA air, things get complicated in the high atmosphere, or at high temperatures, or when electromagnetic fields (caused by thunderstorms, the Earth's core, etc.) come into play.
In the case of water suspended in the atmosphere, it can absorb the radar emissions and thus limit their effective range. However, it does so mostly at certain wavelengths, which are the ones at which water molecules can best capture incoming energy. One such wavelength is the one used in microwave ovens, because food tends to contain water, and heating the water is a fast way of heating the food.
So the ultimate answer to your question is, there is some kernel of truth in those statements, but how large it is is very debatable. Some radars, particularly modern high-frequency ones, operating in the millimetre band, will see noticeable losses due to coulds. Other radars, particularly those in the low GHz range or lower, will fare considerably better, although the lower frequencies have their own disadvantages in terms of resolution and antenna size requirements. Ultimately, defense organizations are quite aware of their equipment's capabilities and usually take steps to mitigate any drawbacks, so it is unlikely that some cloud cover will substantially impair a modern IAD network.

Modi is correct. Adverse weather can affect the quality of the return signal from a radar, and in some cases can mask the objects someone is trying to identify. A good example of this is the Air France flight 447 accident. One of the theories as to why the pilots chose to fly into severe weather is that the area of severity was masked by other weather ahead of it on the flight route, and once they realized what they were flying into it was too late to try to fly around it.

Is this the same case for ground based radar trying to detect flying aircrafts and clouds can mask it?

That theory about the weather radar seems absent from the final report, which does however state: "But the aeroplane had not encountered, before or
during the accident, an exceptional meteorological situation from the point of view
of phenomena that are traditionally avoided in stormy environments" and "The recording of the load factor showed that the turbulence remained light.". Weather is also not listed as a causing or contributing factor for that accident.

Final reports by definition are not supposed to contain theories. I would suggest this as interesting factual material about the weather that confronted AF447... http://www.weathergraphics.com/tim/af447/

@HungryforChallange Radar and clouds don't really care where the signal is coming from.

@JuanJimenez It's a nice analysis, and indeed it was correct in theorizing that icing may have started an issue that cascaded into the crash, but the theory about the radar masking it pure speculation and not held up by the final report, so I question the value of repeating it. The final report does address the case of the weather radar being in calibrated mode and that the captain may have been more concerned if he had adjusted the gain, but at no point is masking considered an issue.

As I said, the job of the final report is not to validate all theories. Neither was it to invalidate them, so the final report really has no bearing on this theory.

@JuanJimenez yet you are using that non-validated theory to support an answer, hence my pointing out that there is nothing to support it. It is about as strong as a  comment on AVHerald.

You are referring to weather radar which is actually designed to detect precipitation. Surveillance radar would be useless if it reflected off clouds and rain.

@AEhere You need to read and comprehend the entire answer, not just the part of it you think supports your argument. You also need to comprehend that this question or answer are not about the AF447 accident. Let it go.

@TomMcW Surveillance radar is useless in quite a few situations, not just weather. That's why there exist transponders (no need for a radar signal at all to determine azimuth, distance and/or altitude) and IFF equipment (enhanced to identify friend or foe), just to name two.

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text:
Why does the co-pilot sometimes keep a hand on the throttle as well as the pilot?
In this YouTube video of a takeoff and landing in a Dash-8, you can see the pilot, while landing, keeps his right hand on the throttle levers.
The co-pilot keeps his left hand at the base of the throttle levers during this time. This happens at 3:38 in the video. I assume this is to ensure both pilot and co-pilot agree when lowering the throttle? I'm just guessing, though.
Why is this, and are there other similar events that happen during a flight?

If it's alright with you, I've added 'sometimes' to the question title, since this is not the standard procedure for all (or even most) cases. It makes sense when flying an approach in IMC, but not in most other circumstances. If you don't like the edit, feel free to roll it back.

The pilots were probably executing a Cat II coupled or monitored approach. The PF is flying the plane (via autopilot, usually), while the PNF had his hands at the base of the throttle ready to take over to initiate a manual go around (or land in some cases, this varies from airline to airline).
There is a thread regarding this on this on airliners.net. It gives one version of the procedure:

At my airline we use this procedure for Cat II approaches. The FO always flies the approach... well the autopilot flies it and the FO monitors the instruments, ready to perform a missed approach if the approach becomes unstable. The Captain at the 500 to minimums call places their hand near the base of the thrust levers, under the FOs arm, and focuses outside looking for the runway. If at any point prior to minimums the captain gets the runway in sight, they call "landing, my controls" and move their hand up to the thrust levers, bumping the FOs arm and hand up and off of the thrust levers. If at minimums and the runway is not in sight the FO performs the missed approach.

It seems like what's described in the quote makes more sense, with the PF being responsible for performing the missed approach and the PNF ready to take over and land if they get sight of the runway before minimums. Why would the PNF take over to go missed when they should be looking out the window?

We back up our PF on the throttles during takeoff and landing on the DHC-6 Twin Otter. It's to ensure that the PF doesn't do anything goofy during an engine failure or other emergency (just a safeguard).

More detail in this answer would be great.

I agree.Should there be any inadvertent throttle retard due to turbulence or runway surface ,the NFP can push them back up.It's SOP in the Navy and Air Force in multiengine side by side cockpits.
It also guards against inadvertent throttle retard due to failure of the seat adjustment slides during the catapult shot( S-3 Vikings,E2C Hawkeyes,C-2 Greyhounds,etc)

Not at all Air Force SOP anywhere I flew Herks.


text:
How is pitch controlled in a flying wing?
How do delta wing or flying wing aircraft maintain their pitch attitude in the absence of an elevator? I understand that the ailerons on the wings do control the banking/roll, but they are differential in nature. Then how can they help in pitch control? For pitch control, both control surfaces need to deflect in the same direction.

You should look at "elevon" on wikipedia: "When moved in the same direction (up or down) they will cause a pitching force (nose up or nose down) to be applied to the airframe"

You already answered: it's enough to move them symmetrically and you get the pitch control.

You basically have the answer right inside of the question (as was mentioned in the comments).  In order to make the plane pitch up and down you have the ailerons move up or down in unison (usually called an elevon at that point, again, as noted in comments.)
Here's a handy diagram to give you some idea of what we're on about:

In the second image the movement of both elevons upwards will pitch the aircraft up as well.  And the elevons going in opposite directions, like the last image, will cause the plane to roll (in this case to the left.)
There are more complex maneuvers as well, like pitching and rolling at the same time. These maneuvers are simply managed by differential deflections of the elevons.  Meaning, if you want to come up and to the left you could leave the right elevon neutral(ish) and pitch the left elevon upwards.  This would push down the left wing, rolling you to the left, and pitch the nose up as well.

Great! Thanks Jay for the wonderful response.
Now I know!

@Romi Sweet, make sure you mark it as correct then ;)

Done! Sorry I didn't know there is an option to mark it as correct. First time! Thanks again!

The reason an elevator can cause pitch up/down is by providing a force, pushing down/up, on a point aft of the Center of Gravity (CG).
A canard provides pitch up/down by pushing the nose up/down (forward of CG).  An elevator provides pitch up/down by pushing the tail down/up (note that pushing the tail DOWN pushes the nose UP).
Are your elevons (combination elevator/aileron) aft of the CG? If so, pushing both of them down will push the tail up and push the nose down.


text:
Is there a software to calculate parameters from certain inputs on a flying wing?
I want to build something similar to this
https://www.parrot.com/de/drohnen/parrot-disco-fpv#parrot-disco-fpv-details
but with two holes in the wing, where I want to put two propellers. But I need to make the proper calculations (shape and size of the wings, I'll work on the propellers separately). It should be a simple design, just a prove of concept.
Is there a software that can help me do this? I can have an idea of the weight I need to lift, the velocity, and need the design (with the two holes).

I see no reason to close this question, it is clear what OP is asking: software for modelling aerodynamics. That exists.

@Koyovis the problem is that recommendations are generally considered off-topic.

two holes in the wing, where I want to put two propellers. But I need to make the proper calculations.": It seems you want to know the diameter of propellers and/or the power of the motors, but it's not clear at all, please edit your question.

After reading a little about Plane Maker, I googled for OpenFoam drone design and found dronecfd, which looks pretty cool. Also found XFLR5. Together I think they can help me to get an idea of things.
Since this are tools written to do what I asked and not part another package that does something else, I think this is a better answers.

Yeah you could start with X-plane, which claims that it can figure out the flight characteristics from geometry parameters. It is based on laminar flow theory and on classic aeroplane shapes and configurations, but it is a start. They have something called Plane Maker which could serve your application.
FlightGear is a similar program, open source so you can study the code to see how it works, then try to find relevant lift and drag data on the interwebs, NASA servers for instance.
Good luck with the modelling effort.

X-Plane also has an airfoil maker that allows building airfoils and/or importing NACA ones. Disclaimer: I work for Laminar Research, the company building X-Plane

X-plane can be tried for basic flight simulations, though it will not be possible to fly the real thing safely just by x-plane simulations. FlightGear does not (did not) have modeling & analysis capabilities. It's a fun and learning experience whichever path taken. Good point in referring to NASA, scholar.google.com can reveal wealth of info on design.


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What is the purpose of aileron droop used in some commercial airliners?
As we know ailerons aid the aircraft to make roll movements and provide lateral stability.
But, what is the intent behind drooping them in some flight conditions, especially when deploying flaps?

If you mean drooping them symmetrically on both sides, it's to complement flaps and increase lift at low speed, but a few commercial aircraft have this possibility. On the MD-11 they are named "deflected ailerons", and, as explained in the video, extending them allows a 3 kt reduction in takeoff speed.

interesting video ! MD-11 is my favorite one , thanks.

The purpose of aileron droop is to use the ailerons as wing flaps. Wing flaps increase the lift generated by the wings at low speed. They occupy a good part of the trailing edge of the wing, except for the outer part, where the ailerons are. By deflecting both the ailerons downwards, they change the wing camber and act like simple plain flaps. They can still act as ailerons, by normal differential deflection.

Image source

so can we call them flaperons?

@Mehdishelby flaperon, elevon and spoileron are relatively extended portmanteaus, but I personaly avoid them unless the surface has been specifically designed to carry out both functions in a similar measure, not as a secondary function alongside dedicated surfaces. You'll still be understood by most people in the industry if you do use them.

You mean ailerons are used as part of flaps with the fowler flaps?

@lemonincider No they're used as plain flaps, the first picture. Not as effective as Fowler flaps but every bit helps.


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Did large winglets contribute to the spin behavior of the SB 13 tailless glider?
The SB-13 swept wing tailless beauty was developed in the early 1990s as a small recreational glider with relatively large upright winglets.  The design proved very spin prone.
It occurred to me these vertical "winglets", set upright, would generate a yawing "lift" component in side slip.  Since these are set behind the CG, the plane would spin INTO the slip (as the change in relative wind increased their AOA).
This phenomena seems to have similarities to excessive spiral instability in a conventional wing/tail design.  Would reducing the size or shape of the winglets help cure this tendency?
Or lengthening the nose a bit?

'It occurred to me these vertical "winglets", set upright, would generate a yawing "lift" component in side slip.' - That's exactly what an ordinary horizontal stabilizer does, isn't it?

RE: winglets, they look like vertical stabilizers (complete with rudders) to me. And being behind the CG, why would you expect them to act any differently from a normal VTP?

Yes, indeed. And forces need to be properly balanced.

One reason for spin issues with tailless designs is that a spin is a stalled condition, and there's no secondary surface that can be forced out of the stall (or prevented from stalling entirely) as is the case with canards and conventional layouts.
Standard spin recovery in most conventional aircraft is to apply forward stick and use rudder to stop the spin -- but with the entire wing stalled, on a tailless design there will be little or now pitch-down authority.  The combination of sweep and large tip fins probably contributes by giving enough continued pro-spin yaw force that rudder (if even present, I'm not directly familiar with the SB-13) can't stop the spin.
So, yes, most likely the tip fins contribute to making spin recovery harder, as compared to, say, all-moving tip rudders, or a coventionally located rudder with conventional fin/rudder proportions -- but they're far from the whole story, as the tailless design itself also contributes.

Probably not. Spins usually start from a stall, not a side slip, and yaw stability generally improves spin resistance and recovery.
For model flying wings, removing the tip fins usually makes spins far more likely and much harder to recover from. The usual explanation is that the end plate effect delays the stall at the wing tip.

Yaw stability, absolutely.  So maybe instead a large high aspect winglet we want more rudder authority. Good point of view from the model side.  Washout or slats are prescribed, as well as "endplates".  But once spin has started, side area imbalance is contributing to the spin.  In other words, without strong opposite rudder, the Vstabilizer actually keeps the spin going.


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Do helicopter rotors have anti-ice systems like propellers and wings?
Icing disrupts the aerodynamic qualities of wings and props, is there a similar danger with helicopter rotors?  Are there de-icing systems to prevent this?

Yes, helicopter blades are no different from wings and propellers when it comes to ice, and there exist rotor de-ice and anti-ice systems for helicopters that require the ability to fly in known icing conditions, typically military types. Some examples include the Sikorsky S-92 and MH-60, and the Kamov Ka-50.
Rotor anti-icing systems are usually electric devices like heated mats, which are mechanically simpler that typical fixed-wing solutions like pneumatic boots and bleed-air-heated leading edges.
Not only the blades require anti-ice systems, however: elements like the engine intakes and air data sensors also need to be kept free of ice just like in fixed-wing aircraft.

Anti-ice systems for helicopter rotors are still not in any widespread usage. This would require a large current supply through some sort of slip ring or somehow directing bleed air to the spinning rotor blades. Leonardo Helicopters has just recently introduced the technology to their line of medium to heavy models. What you see on most helicopters is called a Leading Edge Abrasion Strip. They look somewhat like "anti-ice boots" used on fixed wing aircraft.


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Is pressure lower than ambient on the pressure (bottom) side of an airfoil?
I've been reading about this for quite some time and thought I understood it. Then somewhere someone mentions that air under the wing has higher velocity than ambient thus a lower pressure than ambient. I couldn't find any other reference to this so here I am.
What this means is that, in a purely hypothetical situation, without the upper part of the wing generating a lower pressure zone the airplane would actually be pulled down because the pressure on top would be ambient thus higher pressure than the bottom.
This makes sense since hitting air molecules with the wings will cause them to accelerate and suddenly move faster than ambient, thus have lower pressure.
How accurate is this?
Picture version:

That's why flying too fast would tear a aircraft apart, because outside the skin is fast moving air and inside is non-moving air, so the inside has a higher pressure.

@Sanchises: No, a thick airfoil at low angle of attack has also suction on the bottom, only less than on the top. This question requires a longer answer.

@Peter Enlighten us! ;)

Related: https://aviation.stackexchange.com/questions/13453/what-is-the-actual-air-speed-over-and-under-a-wing-due-to-bernoullis-principle

If the airfoil generates lift, pressure on the upper side has to be lower than on the bottom. Still, there can be suction (less pressure than ambient) on both sides.
For an extreme example, take the airfoil from US patent application 2004/0094659 A1 by Dan Somers. It tries to stabilize the boundary layer over the first 80% by creating an increasing suction over chord. The picture below is shamelessly copied from this application.

Ambient pressure is at a pressure coefficient c$_p$ of zero, and negative values denote suction, i.e. lower than ambient pressure.
All lift is generated by the highly cambered flap while the forward wing will only start to contribute lift once the angle of attack is increased from the current value of -2°. Note that airfoils for high subsonic speed show the same philosophy of a rooftop pressure distribution over their forward part with high rear loading. Such airfoils exhibit a high pitching moment and require a larger tail surface for stability, which blunts their advantage of low drag a bit. The plot below is taken from US patent 3,952,971 by Richard Whitcomb and shows just such a transsonic airfoil with a weak shock on its upper side.

This plot shows the speed difference ∆v compared to ambient, and again higher speeds which equal more suction are positive.
All those airfoils will develop a suction peak at the forward end of the upper surface and a corresponding pressure increase on the lower side once the angle of attack is increased. Now a Birnbaum-type pressure distribution will be added and at high angle of attack, this effect will dominate the pressure distribution and will ensure higher than ambient pressure on the lower side. This effect will be the more pronounced the thinner the airfoil is - a thick airfoil exhibits a stronger displacement effect which adds a little suction on both sides.

hitting air molecules with the wings will cause them to accelerate

Not exactly hitting - it is air molecules flowing around the wing which will contribute a speed increase and hence a lower local pressure. This can best be seen by the pressure distribution around symmetrical airfoils at zero angle of attack. Note in the plot below that suction is on both sides and growing with airfoil thickness. Of course at that angle of attack those airfoils do not generate lift, but when an angle of attack is added, they will, and it will need a higher angle to push the bottom side of the thicker airfoil into the pressure region of negative c$_p$ values.

Inviscid pressure distribution of symmetrical NACA airfoils (picture source).

Ambient pressure is higher than any wing pressure

Not quite: Note that all plots show a pressure peak at the nose and a pressure recovery to slight overpressure at the trailing edge. While most of the pressure around a thick airfoil at low angle of attack is indeed lower than ambient, the stagnation point will always ensure that this is not true for all of the airfoil.

I think the first word should be ‘yes”, not ‘no’, because both drawings in the question show the pressure being lowest on top and the question is just whether the pressure on bottom is still lower than ambient, which you confirm.

@JanHudec: Initially I wanted to write "it depends". You are right, a No is clearly misleading. But the new understanding is also wrong - there is always a stagnation point.

@ymb1: It all depends on the point of reference. If the wing is stationary, the molecules flowing towards the stagnation point will be decelerated and slowed down. If the point of reference is stationary and the wing moves, the wing will add some energy to the stagnation point flow and increase pressure. And yes, the air will be accelerated forward first, backwards after that in the suction region and slowed down again when it leaves the airfoil.

@PeterKämpf I tried explaining the frame of reference point on a question about static ports a year or two ago and got a ton of down votes, people just can get past thinking about the aircraft as stationary and the air flowing at speed. It could help your answer if you explained more how positive Cp's are higher than free stream static pressure.

So that airfoil is just a fairing for a beam, with a flap at the back


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Force Balance in Wind Tunnel
I am considering a model wing inside a wind tunnel supported from underneath from its center of mass.
Two load cells can be used, when properly oriented, to measure the lift force and the drag force. How can two load cells be arranged to also measure the pitching moment and specifically the constant aerodynamic moment I discussed in other threads?
Thank you!

So you want to measure two orthogonal forces - this requires two sensors already. There is no degree of freedom left to measure pitching moment, too. It will be best to do two identical runs and measure one first and the other in the second run.

Yes, i have two load cells and I am learning how to use them. One for lift and one for drag (orthogonal to lift). trying to see what arrangement (separate experiment) I need to put the two load cells to measure the aerodynamic pitching moment. I will only place the wing model (not the fuselage) in the test area.

I had imagined simply mounting your lift and drag sensors on a simple (HO gauge model railroad) track for a DIY homebuilt wind tunnel.
Pitching moments for your wing could be done as a separate experiment.  Your could pivot on your aerodynamic center (as an axle) and measure pitching torque moments with the load cell attached to the end of the fuselage.  Once these moments are established, you can then design the H stab based on area and distance from your wing.  Ideally, the H stab will keep your pitching moments zero through all angles of attack (including stall), leaving pitch control to the pilot (with the elevator).
Similar study can be done with you center of pressure (center of all lifts), once you establish it in your planform design.


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Are the pitching moment and pitching moment coefficient of an airfoil given w.r.t. the aerodynamic center?
When the magnitude and sign of the pitching moment and the value of the pitching moment coefficient are communicated, are these values given with respect to the aerodynamic center of the airfoil since the magnitude of the moment of a force is always dependent on the chosen pole O about which the moment is calculated (unless we are discussing pure moments which are independent on the pole O location).
Thanks!

A moment is a free vector. There is no need to add a reference point - it can be shifted fore and aft or up and down without changing its effect. This effect is a rotation without translation.
So yes, as you suspect already, the pitching moment is a pure moment.
If there is also a force around (like on a wing), you have two vectors: One for the force and one for its moment. Put the reference point along the vector of the total force, and that moment disappears. Shift your angle of attack to the zero-lift point, and the force will disappear, leaving you with a pure moment (which is the sum of all the remaining forces along the chord which balance at this angle of attack).
In two-dimensional potential flow theory, all angle-of-attack-dependent forces act on the quarter point. If you now define your reference point at this quarter point, all variations in lift with angle of attack have no moment around this point! What is left is the pure pitching moment which does not depend on angle of attack.
If, however, you choose some other point as your reference point, the change in the lift force with angle of attack will add a moment, so your pitching moment is some constant value plus an angle-of-attack-dependent contribution. If you pick your reference point behind the quarter point on an airfoil with positive camber, there will be one angle of attack where those two moment components balance and the pitching moment will disappear. Then your reference point is the center of pressure at that angle of attack.
So the split of the aerodynamic forces into a force and a moment is purely arbitrary. Technically, we could always sum up all forces in the center of pressure, but then we need to keep track where it is, because it moves with angle of attack. Therefore, we decided to make things a bit more complicated and express the aerodynamic forces as one force at the quarter point and a separate moment. Why the quarter point? Because then this moment does not change with angle of attack. That is the advantage of defining all forces in the neutral point. We always know where the force acts at the price of carrying an extra moment with us. At least, this moment is constant over angle of attack - that is the big advantage of choosing the neutral point as the reference point.

EDIT
When looking for older answers to link to, I found that the two of us are discussing pitching moments since years without getting anywhere. Like here, here or here. I'm afraid I will repeat the same stuff like a broken record, and you keep asking the same question over and over again. Maybe it is better that you explain in a new question what your current understanding is so I can work out where exactly we two talk past each other.

But for a fixed $AoA$ and a given lift force $L$ which acts at the $CP$, if the pole $O$ is the located at the LE or at the TE or elsewhere, the lever arm $x$ changes and the moment $L*x$ varies too, or doesn't it?

@BrettCooper: That would be the lever arm of the lift force, and that force isn't a free vector. Of course this lift also has a moment, and this moment changes when the vector is shifted perpendicularly to its direction. Now split that in two: A pure force (bound vector) and a pure moment (free vector). The question is about the pure moment.

I was thinking that the pitching moment was the "total" moment $M_{total}$ acting on the wing. This total moment has two contributions, one that is independent of lift $L$ and is a pure moment, and one moment contribution that is due to $L$. Since the moment portion due to $L$ depends on the lever arm $x$, the total pitching moment $M_{total}$ would also depend on $x$ whose value depends on where we choose the pole $O$ to be...

@BrettCooper: That is only a matter of definition. Of course you can split any moment into a free and a bound component, but that is more complicated than having only a force and a free moment. I think I'll add some more lines to the answer.

Thanks. I created a new question to better address my dilemmas...


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Any biplane flying wings after 1914?
Were any biplane flying wings built and flown after the 1912-1914 variants of the Dunne D.8? If not, why?
Wikipedia mentions merely that this type was too stable and insufficiently controllable, but that's not inherent to this layout.  And the biplane's stiff truss structure remained useful for the next half decade.

Maybe use "blended-wing" as a tag.

BWB's belong to the jet age, unfortunately too fast for biplanes.

See this question over on meta. Maybe a new tag is coming soon...

The Burnelli RB-1 and RB-2, 1921-1925, are biplanes with lifting bodies, which is sort of adjacent.

Yeah, more of a lifting body with a vestigial stab and elevator than a true flying wing.  Clever layout for an airliner!

It is arguable that these planes are not true flying wings because things like the pilot and engine sit or dangle outside the wing airfoil. Besides Dunne's D.1, D.3, D.4, D.5, D.8 and D.10 of varying success, W Starling Burgess in the US built 19 Burgess-Dunne tailless biplanes, many of them waterplanes, e.g. the 1916 AH-7.

The 1930 Smith B2 Arrowhead was a similar landplane, backed by Glenn Curtiss (who had bought out Burgess).

The 1932 Westland-Hill Pterodactyl V was a tailless sesquiplane - a biplane with the upper wing bigger than the lower one. It flew well enough as a fighter prototype but, with the revolutionary fast monoplanes beginning to prove their worth, was obsolete before it flew.

The Easy Riser has already been mentioned.
Can't recall any others offhand, I might have missed something.

The Pterodactyl series is a fascinating set of experiments!

Might the narrow CG range of a flying wing (biplane or not) explain why this type was so rare?

Yes. Also, the stability of a practical tailless wing is a complex and subtle issue which many designers get wrong.  This link gives some related information: https://aviation.stackexchange.com/questions/62377/why-are-there-no-cargo-aircraft-with-flying-wing-design/71734#71734

OK, I found one, from a friend's suggestion about hang gliders.  The late 1970's UFM Easy Riser, initially a go-kart engine strapped to a foot-launched hang glider, the "first ultralight."
But this is even more Octave Chanutesque than the 1912 Dunne!  Are there any, shall we say, nonprimitive models?  If not, why?

Ah, now that we have a tag flying-wing, maybe someone knowledgeable will notice and answer.


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What were the disadvantages of early prop driven blended-wing aircraft?
I've been researching the history of pre-jet aircraft and I've come across several famous, and not so famous, blended-wing aircraft. The idea seemed to show promise but never really caught on, even for a limited span of time or confluence of technological factors e.g. weight vs engine power etc.
Just to be clear, I'm not interest in flying wing and/or tailless aircraft whose early problems are well known, but rather those designs that have tails but have a fuselage shaped to produced a significant component of lift.
Just a few of the designs I've stumbled across:
Burnelli CBY-3
Cunliffe-Owen OA-1 (Burnelli license)
French de Monge 7.4
Soviet LK-1 a.k.a. Fanera-2
There are quite a few more I don't have handy web links for. Junkers did a couple that would probably qualify.
The large number of designs from all over the world show that the basic idea offered promise but all the designs eventually went no where despite being tried in many countries, economies and even political systems. That in turn seems strange because in the 1920s-early-1940s, lift seemed more important than speed for larger multiengine aircraft i.e. just getting a load airborne at all was worth quite a few other tradeoffs. As engines grew more powerful that tradeoff lessened but for at least 15-20 years a high lift design should have been more desirable than a high speed one for larger aircraft.
The failure of many different designers over the course of three decades to make the blended wing work suggest to me that the basic idea has some inherent disadvantage/s that outweighed any gains. Unfortunately, none of the resources I have consulted offer any clues to why the designs did not "take off."
I suspect the designs showed some rapid onset of stall of instabilty but that's just an uninformed hunch on my part.
Any clues?

At speeds usual in 1920s-1940s, there is really no trade-off between speed and lift. In either case you need high lift-to-drag ratio. For that you need high aspect ratio and since fuselage is long, making it a lift-generating surface decreases the aspect ratio, which might be relevant.

also, the control problems associated with flying wings would exist, at least in part, also with blended wing designs. It wasn't until the discovery of things like elevons that aircraft without separate elevators and ailerons became feasible.

It would seem that having in effect two wing geometries, one thick with a long chord and the other relatively thin with a short chord, would give two wings with different lift, drag and stall behaviors. It's like a frozen variable geometry aircraft with both extremes of geometry in operation at the same time. I suspect the planes had hidden "coffin corners" in their flight envelopes but I can't be sure.

@jwenting: Elevons was invented very early in aviation. Dunne had a glider with elevons flying in 1907 - just 4 years after the Wright Brother's first flight and flew the first tailless with elevon control in 1910. He didn't call them "elevons" at the time. His patent drawings refer to elevons as "flaps".

Generally, to create lift means to deflect the oncoming airstream downwards. How and where this is done in detail on an airplane is less important than doing it with a smooth spanwise lift distribution such that the wake behind the aircraft is trough-shaped.
A fuselage is disturbing this lift distribution, but not severely. The length of the fuselage means that it creates considerable lift at small incidences, even with the low lift slope of slender bodies. In case of the N-250 it was found that an incidence of 2° produces the lowest drag. However, this also means that the lift is only just right at one angle of attack, and when the aircraft is flown in different conditions, the fuselage can mess up the lift distribution quite easily, especially if it is wider than necessary.
During the 1930s and 40s, when this was understood in detail not only in academia, but also in the design offices, progress in engine technology pushed operating altitudes up until planes needed pressurized cabins to transport passengers comfortably. Pressurisation demands a round or nearly-round fuselage cross section to keep structural mass down, so any attempt at widening and shaping the fuselage would result in an unacceptable mass penalty. This ended the experimentation with lift-creating fuselages.
As for the N-250, the idea of fuselage inclination in cruise is correct in theory, but resulted in massive protests from the cabin crew which now had to push their carts uphill, ending this kind of optimization quickly. This is why it never caught on.

Let me make sure I understand: 1) The relationship between lift and angle of incidence is dependent on chord length. 2) The fuselage being long, regardless of shape, has different lift to angle ratios than the short chord wings. 3) In a plane with a wide fuselage, the delta between the the changes in lift contribution between fuselage and wing as the angle of incidence changes will be larger than on a narrow fuselage.

...The early wing shaped fuselage craft would in effect have two separate wings, one a long chord, the other short, each producing different lift at significantly  angles of incidence. As the plane changed orientations,the contribution of lift would slide about the between the wings and the fuselage. I presume this would manifest as poor handling with sudden shifts response as the orientation changed?

...Also, does chord length have much to do with stall angle as well? If you have to wings one long one short, and they stall at different angles. That couldn't have made the plane easy to fly.

Is the fuselage lift the reason that STOL aircraft like the C-130, have such a stubby appearance. I'm sure it helps with cargo capacity but it you need a wide fuselage then you need long chord wings to keep in sync with the lift profile of the fuselage, correct?

BTW, hope I got the vocabulary right. Educated as biologist, spent my life programming. Not a specialist in the field.

Flying wings, or all wing craft wouldn't feel the effect as strongly because the change in chord length would occur gradually as the wing swept back.

@TechZen: Please post your questions not as comments, but as a new question. This makes them easier to answer. Very generally: Lift curve slope depends on the aspect ratio, the ratio of span/width to chord length. Handling is also influenced by the position of the lifting surface . Surfaces with a low aspect ratio have less lift at the same angle of attack, but stall at a higher angle of attack. But again, please post a new question, and I will give a comprehensive answer.

Will, do. I don't know enough about the subject domain to know when a clarification request breaks out into its own question.

I remember having read that in these airplanes, at high speeds and low angle of attack, it was the center section, the lifting fuselage, that provided most lift, but at an steeper attitude, it was the regular wing that lifted. The shape of the 'lifting fuselage' sides makes suspect an increased 'marginal vortex' drag, this reducing the efficiency of idea. For a new procedure to be added to series, at least in the Automotive industry, they say it must provide a 10% increase in efficiency, along with a 10% reduction in costs, this is why changes that a private owner can introduce in their engines or vehicles may be worth for them but not for a company.
The mail boxes open to suggestions by people in the production lines, that once received concepts such as the way to use a single screw or bolt instead of the several previously used, saving the cost of thousands of bolts a year, are closed now, changes in the production line for this minimalist improvements were too expensive to implement in recent times.
Besides this, a patented idea must offer an overwhelming advantage for a builder to pay for it and incorporate to their production, if improvement is not big enough as to offer noticeable gains from the patent, the time will be left flowing, when the patents have expired, in most cases advances in technology offer better things.
However, some XXI century airplane designs incorporate concepts analog to the Louis de Monge and Burnelli's lifting fuselages. The time will say if it are good enough. Salut +

Looking through your first link about the CBY-3, it seems there may be political reasons for the failure of this design. To this day, many organizations still refuse to acknowledge the merits of the design. Part of the issue is probably the interests of much larger companies which were already deeply entrenched by this time.
Your second link is related to the Burnelli designs, and the page also offers this explanation:

The Cunliffe-Owen OA-1 Flying Wing was almost certainly too late to be a success in the civil marketplace. By the time it was ready to fly it was already 4 or 5 years behind the times in terms of performance and economy.

In my studies of technological history, I've never found an example of a suppressed technology. I think its a myth that it ever occurred. There are simply to many ways to route around any attempts at suppression, not the least of which is multiple political jurisdictions with competing self-interest.   It's more likely that powerful interest will use political or market power to steal a new idea than to suppresses it. I think the myth is evoked to hide either faults in the technology or its implementation.

I am aware of the contention that Burnelli failed through hostility from FDR but that doesn't explain why the design didn't succeed in Nazi Germany or the Soviet Union either. Likewise, Burnelli's patents were not extensive enough to cover all lifting fuselage designs of the era, even just in the US. Since all the sources on this technology are de facto marketing it, they don't cover any disadvantages at all. Since all designs have a tradeoff that produces a disadvantage, that means the disadvantages of the blended wing back then were either not understood or have been hidden.


text:
Why are there no cargo aircraft with "flying wing" design?
From that I have seen so far, the "flying wing" design (like the one of B-2 Spirit and Northrop YB-49) has superior performance but also a few notable problems that make it difficult to use for passenger aircraft:

It is difficult to control, and the YB-49 crashed even when flown by an elite test pilot. However, computer assistance has been implemented for B-2 and I do not think this is a problem any longer.
There are problems related just to the passenger transport: not enough windows, difficult to evacuate.
It also cannot be pressurized as easily as a cylinder but for a majority of possible cargo this is probably not a problem. Some cargo may not require pressurization at all and some may only need partial pressurization like in jet fighters.

Hence I understand that there are problems on the way to the flying wing passenger aircraft. However, why there are no cargo aircraft of this kind around?

Very related: Why are there so few aircraft that had inhabited wings?

"computer assistance has been implemented for B-2 and I do not think this is a problem any longer" Boeings have been computer assisted for years (decades) and even they still have problems. An airplane being computer assisted isn't the end of all problems and it's not a magic bullet.

"It also cannot be pressurized as easily as a cylinder but for a majority of possible cargo this is probably not a problem" True, but since you mention passengers a couple of times, it's quite a problem for passenger transports at those operational heights and speed.

Flying wings can be made to have acceptable flying qualities without any artificial assistance.  Just look at the Jim Marske glider designs.
The principal downfall of flying wings is that stability in pitch is pretty much achieved the same way as with a conventional tail, with a down force balancing out the center of gravity forward of the fulcrum of neutral point of the lifting forces, but it's all being done over the very short moment arm of the wing chord itself.  In other words the "tail" has been moved forward to the trailing edge of the main wing.
There are a lot of issues that result from this, pitch sensitivity and damping issues and all that, but the biggest one from a cargo aircraft's perspective is a very narrow center of gravity range.  Not a big deal on a bomber with a concentrated bomb bay load, or a glider that doesn't have to cope with loading variations, but a bigger deal on a freighter.  You are forced to spread the load, and the fuselage volume, laterally, creating way more frontal area than necessary (you're in effect turning the fuselage sideways), so you end up cancelling out the drag benefit of doing away with the tail in the first place, and still end up with a "temperamental" configuration.

Admittedly, without a long fuselage there will not be much length along which the cargo can be distributed. I'd call it a wash.

That's what I meant by having to spread the loading laterally.  But even within the space envelope you would have just within a flying wing stump fuselage or center section, the available loading range is pretty narrow.  Bring your knees to your chest in a FW glider, where the allowable range is couple of inches, and you might find yourself aft of the rear limit.

most excellent explanation!

This explanation is plainly wrong, as stability is not due to this. an aircraft can be perfectly statically stable with the center of lift ahead of the center of gravity. - In fact many aircraft work that way and it's more stable that way. - This is due to the lifting moment, and the way cl-alpha works.

Admittedly it's better to use "neutral point" which takes in all of the various forces and moments acting on the aircraft that influence the "net balance point" so to speak, instead of center of lift.  There still has to be a net down force acting at the tail, balancing the CG that has to be forward of the neutral point.

@paul23 the "flying wing" will still have a narrower CG range.  And the point about "turning the fuselage sideways" is also correct. But good to hear from someone about the weight ahead of CG/down force on tail design. This ties in well with the concept of making tails smaller in a safe manner, rather than stalling tiny Hstabs in down wash because weight is unnecessarily too far forward.

@RobertDiGiovanni the conclusion is indeed correct, the reasoning is not. However to explain I'd have to write several pages or assume high level education in physics.

Which conclusion, which reasoning, and which stability?  "Positive static stability", as you point out, is not the same as directional stability.  Nuetral static stability is the lowest drag configuration.  Smaller tails need stricter CG limits.  I'm going between conventional and tailless, as the trend is for smaller tails in airliners.  It needs to be done right.

https://en.wikipedia.org/wiki/Longitudinal_static_stability As you can notice in the image under analysis there, an aircraft can be in equillibrium when the lifting force is in front of the center of lift of the main (and tailing) wing. - Since the lifting moment counteracts the moment generated by the lift-gravity pair of forces. - Stability is not a problem due to this simple view. The problem is that when a wing increases angle of attack the change in total moment needs to be either negative or zero (to be "stable" something needs to counteract changes).

This means that dM/dCL < 0. From this calculation the range where the center of grativity can be, can be calculated. The wikipedia page I linked has the equations. as I said going into detail to explain these requires more than I can do here.

Longitudinally stable: when it is pitched up, the tendency is to pitch down again.  The weight forward/tail down force school is: pitch up, slow down, nose down, recover.  When a wing increases AOA and Clift of wing changes, the added tail moment of a properly designed Hstab keeps net Clift in same spot.  Nothing to do with CG.  A (low drag) flat plate of adequate area will do this.  Notice that the "down force" vanishes at 0 AOA, when it is doing its job holding wing AOA at proper angle.  Low powered, slow plane designers knew much about this 100 years ago.

@RobertDiGiovanni First of all stability is all about the location of center of gravity, secondly the aerodynamic center (what you call center of lift), is actually -by definition- the point on a wing where the angle of attack (alpha) does not change the coefficient between moment and lifting force. It does not change with angle of attack. Did you read the wikipedia page on the longitudinal stability?

Yes.  Sadly, trim is all about CG.  Ideally, CG belongs directly under Center of All Lifts (Not aerodynamic center) in flight. The misconception is that Hstab decalage "downforce" balances forward CG is not good design.  The Hstab sets wing AOA.  Plane A is 500,000 lbs loaded, plane B 600,000 lbs.  In order to fly same indicated airspeed, plane B needs a higher wing AOA.  So, more angle of decalage is added to Hstab.  In any type of plane, abuse of CG limits is not good. I would not go desperately cranking my Hstab to fix it. But thanks for your input and point of view.

Would a ground effect vehicle be more stable as a flying wing?

@paul23 lets go with the term neutral point.  Are you saying that the CG can be aft of the neutral point?  So that the horizontal tail is lifting?

@JohnK Well of course not, since the very definition of the neutral point is the point most to the tail of the craft where the center of gravity is located keeping he full aircraft stable. - The actual explanation I did in an answer below: your conclusion is as I said correct just the reasoning isn't. (It's not about the simple balance of 2/3 forces).

John K: You repeat an old misconception - with cg at the neutral point all surfaces contribute equally to lift. Stability does not need a tail downforce. @paul23: That is easy to explain on one page and does not need a high level education in physics. Maybe you are just a poor explainer.

I'm not sure where you get that Peter.  My opinion is that since the aft CG limit on any aircraft is x percent minimum forward of the neutral point, on any normally loaded aircraft there is always a minimum nose down pitching moment present, the torque applied by the CG acting on the neutral point, with the tail providing some minimum of down force to oppose it except in transient conditions in maneuvering.

Cargo aircraft (outside the military) almost always started life as passenger aircraft. The ratio of active large cargo aircraft to passenger aircraft is in the single percentages. Therefore, nobody develops a pure cargo aircraft from scratch.
That does not mean that no one has tried. Especially for cargo, large flying wings have been proposed which store their cargo in containers along the wingspan - hence their name: Spanloaders. Below is an artist impression from the 1970s.

Boeing Model 759-159 distributed load freighter concept from the 1970s (picture source)

And to the military, soldiers are just another kind of cargo.

Where would that thing park?

Not at the airport it's flying over, certainly...

Perhaps it doesn't park, or even land - just flies endlessly while smaller craft ferry fuel and cargo between it and the ground.

@RogerLipscombe; Actually, if you look closely, you can see two of them on the ground. But I agree, a standard 98 ft runway will be too narrow.

In addition to the other answers, a reason for the lack of flying wings in civil aviation in general is that they need to compete in an environment that has grown alongside conventional, fuselage-and-wings aircraft and is ill-suited for flying wings.
This means they need to use the same airports (turning radii, RWY widths), fit into the same parking envelopes (wingspan) and be serviced by the same ground vehicles (bay heights, wing clearances). Because redesigning an entire industry worth of ancillary equipment and infrastructure has been deemed not worth the minor efficiency gains to be had from flying wings.

and the extremely conservative attitude of the people making purchasing decisions, that makes it very hard to get even things that look or sound a bit different from the established norm from getting adopted (think the Boeing Sonic Cruiser concept, or the Beechcraft 2000, as prime examples).

For a start, with what it costs to design and certificate a new aircraft type, if a transport craft can't be reconfigured to carry either passengers or freight it won't make it off the napkin.  The conventional transports we have can be switched from cargo to passenger and back, some in just a few hours.  For a non-passenger transport to compete, it would have to be much cheaper (to buy and to operate) than a multi-purpose airframe.

Flying wings simply don't have much internal space for cargo, so they're a non-starter for cargo planes.
You mention the B-2 which will carry 18 tons of bombs.  However, bombs are small and heavy: for example, a US Mark 82 bomb is essentially a 130kg (300lb) metal box filled with 90kg (200lb) of explosives. Most airline cargo isn't packed in thick, heavy metal boxes like that, so turning the B-2's bomb bay into a cargo bay wouldn't create a very useful cargo plane.
Which is good, because the designation C-2 is already taken. *rimshot*

"In 1974, DARPA requested information from U.S. aviation firms about the largest radar cross-section of an aircraft that would remain effectively invisible to radars. [...] Lockheed had experience in this field due to developing the Lockheed A-12 and SR-71, which included a number of stealthy features, notably its [relatively short] canted vertical stabilizers" https://en.wikipedia.org/wiki/Northrop_Grumman_B-2_Spirit - All stealth aircraft have two tiny stabilizers or none. If stealth isn't your goal then prob don't try so hard at $2B per plane.

I would like to discuss the stability argument in a bit more detail. Since it is correct that static longitudinal stability is the main reason why these aircraft are not often developed.
However the reasoning given in the other posts is incomplete/not completely correct.
First of all, a flying wing indeed has a very small stability margin. This can be solved by either some unconventional wing designs: this has the problem of defeating by large the efficiency gain of using a flying wing configuration.
The other method, employed by the B2 spirit is to use an active controller to control control surfaces. This has the drawback of increasing complexity of the aircraft and passing regulations tests is even harder.  some reference.
Static longitudinal stability
I'm going to explain the static longitudinal stability in detail a bit more. First we define stability: to be stable means that whenever a small excitation is applied to the object, the object will "recover" itself.
Longitudinal  stability means that an excitation in the longitudinal direction, thus a change in pitch/angle of attack ($\alpha$), needs to be countered by "some" moment. Since an aircraft during cruise in equilibrium, an increase in angle of attack, should lead to a negative moment. - A reduction of angle of attack should lead to a positive response moment.
Or in a mathematical way: (definition)
$$\frac{\partial M}{\partial\alpha} < 0$$
A simple wing
Now let us first look at a simple configuration: just a wing. Since lift generated from a wing is due to a distributed force, a wing will always have both a Lifting force, and a lifting moment (except at a single point where the moment is zero, however this point changes with flying conditions). - In aviation we remove the units for simplicity's sake. So we have a force $C_L$ and a moment $C_M$.
On an airfoil there is also a point where the factor between $C_L$ and $C_M$ doesn't change with angle of attack. This point is called the aerodynamic center and is a static point given by the airfoil shape: it is hence used to calculate.
So (by definition):
$$\left( \frac{dC_m}{dC_l} = 0 \right)_{a.c.} $$
Now since a wing always generates more lift under a higher angle of attack, and actually we consider the C_L - \alpha curve to be linear. (For stability we consider small changes in angle of attack) the following holds:
$$ \frac{d C_L}{d \alpha} = C_{L_\alpha} > 0 $$
Together with the earlier equation:
$$ \frac{d C_M}{d \alpha} = C_{M_\alpha} > 0 $$
conventional aircraft
I first wish to address the stability of conventional aircraft in this point, as there seems to be a lot of contradicting information.
For this consider the following configuration (notice that the points where the lift "attaches" to the wing & tail are defined to be the aerodynamic center for these calculations - we could use any point, but using ac reduces complexity a lot).

From the static equilibrium equations:
$$W = L_W + L_t$$
$$L_W = \frac{1}{2}\rho V^2 S_w \frac{dC_L}{d\alpha}(\alpha - \alpha_0)$$
(above is just the lift equation, which defines $C_L$)
The lift due to trim in the tailplane is more complex (due to the non negligible down wash of the main wing on the airflow at the tail (${\epsilon}$). ($C_l$ = lifting coefficient of tail section)). - Simplifying, we consider the horizontal tailplane to be a symmetric airfoil, so lift at $\eta=0$ is zero. (of the tailplane).
$$L_t = \frac{1}{2}\rho V^2 S_t \left( \frac{d C_l}{d \alpha} \left( \alpha - \frac{d \epsilon}{d \alpha} \right) + \frac{d C_l}{d\eta}\eta \right)$$
Similarly the moment equation can be written:
$$M = L_Wx_g - (l_t - x_g) L_t$$
Now from the very first equation again, the partial differential of the moment equation with respect to the angle of attack needs to be negative:
$$\frac{\partial M}{\partial \alpha} = x_g \frac{\partial L_w}{\partial \alpha} - (l_t - x_g) \frac{\partial L_t} {\partial \alpha}$$
Now there is a final definition that needs to be made, a distance $h$ from the center of gravity so that for the total wing the moment equation can be written as:
$$M = h(L_w + L_t)$$
Solving all equations (see wikipedia for details) leads to:
$$h = \frac{x_g}{c} - \left( 1 - \frac{\partial\epsilon}{d \alpha} \right) \frac{C_{l_\alpha}}{C_{L_alpha}} \frac{l_t S_t}{c S_w}$$
With  $c$ being the main aerodynamic chord of the main wing. (Introduced once again to reduce the amount of units we work with). For stability (since $C_{M_\alpha}$ needs to be negative) $h$ needs to be negative. Let's analyze above result:
$$\frac{l_t S_t}{c S_w} = V_t$$
This part, called the "tail volume", consists of geometric definitions of an aircraft and won't change.
$$1 - \frac{\partial\epsilon}{d \alpha} $$ are the stability derivatives and difficult to calculate, but typically found to be at least $0.5$.
So this allows us to define the stability margin as:
$$h = x_g - 0.5cV_t$$
Note that since the second term is always positive, having a negative $x_g$, or (see image above) having the center of gravity in front of the aerodynamic center of the main wing. will always give a stable configuration. And remember that aerodynamic center does not change with angle of attack. (Center of gravity can shift during cruise due to fuel consumption, but this is typically mitigated in practice by pumps, and shifting center of gravity forward will always give a more stable aircraft).
neutral point
Now finally we are at the neutral point, which was used in another answer incorrectly consistently. The neutral point is, by definition, the point at which an aircraft is "just" stable: $h=0$
$$x_g = 0.5cV_t$$
From this it follows that the "range" between which the center of gravity can change is between nose of the aircraft (negative $x_g$) and a point given by mainly the tail volume. The tail volume is most easily influenced by changing either the tail surface or distance between main wing and tail.
Flying wing configuration
Finally back to the original point, the flying wing configuration. A flying wing, by definition, has no tail behind the main wing. Thus the tail volume is zero.
Hence the neutral point of a flying wing is exactly at the aerodynamic center. Which is for a conventional wing design about 1/4th of the chord distance.
thus a flying wing has, without modifications, an unusable small stability margin
Delta wing and canard
I'd also wish to quickly sidestep to the delta wing and canard configuration such as for the concorde or f16. These designs are driven by another parameter (shockwave drag/something else, like more efficient control due to no downwash).
However the stability for such aircraft is a lot different: while the picture above can still be used, we need to consider that $l_t$ is, by design, negative. This changes the location of the neutral point to always be in front of the main wing. And many of those designs also have active control surfaces and are inherently unstable.
(The name "canard" even came from this: when the brother wright created the first powered aircraft, in France people didn't believe it. They called it what we would call today "fake news". The term for fake news was "canard" in France, so they called the design a "canard").

Off to a good start.  Now, "tailless" consideration.  Make wing chord longer (lower AR).  More stable (slower rate of pitch). Now extend a portion of the wing forwards and backwards ( fuselage) even slower pitch.  Now flatten the rear portion of the fuselage. (Even more stability). "tailless" flying wings use the trailing edge as a "tail". It is just not as effective as a conventional one for trim when CG is not directly under C all lifts.  Hang gliders illustrate this.  Weight forward will make an arrow more stable. When a wing is added, CG and Clift imbalance must be trimmed. +1 4 U.

Using the trailing edge as a tail doesn't change the fact that for stabilit the tail volume is zero. - This is an aerodynamic property and not a property of the aircraft. It is indeed often done, as I said in the opening paragraph. The effect of this is that the aerodynamic center moves backwards (remember the definition of the aerodynamic center). It's hard to predict the effect without going into CFD though.

Trim on the horizontal control surfaces, is however of no influence for the stability, without correct trim settings the aircraft is still (most often) "stable". It's just in a stable slope that either increases or lowers altitude: that is still stable though. (Might not be what you want at that moment, but that's for the pilot to decide not the aircraft design).

The "fake news" sense of canard is very recent. Having the control surfaces at the front was called a canard configuration not because it was unbelievable but because it was first used on the Santos-Dumont 14-bis, which was said to look like a duck ("canard", in French) in flight. Also, neither Concorde nor the F-16 has canards.

I'm sorry, I lost the point that you were trying to make.

@Koyovis, this is in response to the most accepted answer - which is blatantly wrong. Physics do not work the way described in that post, it is NOT correct and I have no idea why people keep upvoting it. This post tries to solve that by trying to formally show the calculations.  - So I'm not trying to show a direct answer, rather I am showing the calculations so anyone can come to their own answer. (Which hopefully is equal, given the input and calculations are equal).

It's all about CG range and how much abuse the design can take.  Take a look at the C-130 Hercules.  It has a huge Hstab to cope with a wide range of CG.  Really a bi-plane.  So is the Chinook helicopter.  Holding the table up with 4 legs (6 with a canard).
So, what do we do to get to a viable flying wing? Sweep back offers improvement in pitch stability as (with washout) you lengthen the aircraft.  Control surfaces can be placed at the wing tips.  Reflexed camber airfoils also help.  How to cope the loss of a longer fuselage/Hstab pitch torque arm?  Have the cargo bay set on a roller at CG.
Pull it forward until it tips. Secure, cargo balanced!  Fuel tanks can be arranged to drain evenly.  Assuming a subsonic design with near neutral static stability, it may even fly without computers.
But the all important shift in Clift with change in AOA or airspeed must be accounted for.
So a small tail, like birds have, may help build a better safety margin for the design, with or without computers.  Ditto for lower aspect wings.  Interestingly, a bird sweeping its wings back becomes ... a delta.  Sweep them back out ... an F-111?
It is possible to reduce tail size in cargo, and passenger planes.

The first certified stable aeroplane flew in 1910 at (and without) the hands of J W Dunne. It was also the first tailless swept plane to fly, sort of a biplane flying wing except that everything was piled in between the wings so not a true flying wing. Contemporaries Handley Page and Igo Etrich were obliged to add tails to their more bird-like attempts. Whether a given tailless type is adequately stable is complex and subtle to analyze and many designers have since got it wrong. In 1913 Dunne lectured the Aeronautical Society in no uncertain terms on why his worked and the others failed, it makes a fascinating read even today.
But all have been agreed that the subsonic tailless type has a narrow CG range. That is not a problem provided you do your load balancing properly, but it does make the chore even more awkward than usual.
The real killer for cargo planes is that the hold of a flying wing only gets deep enough to be practical on a huge design, otherwise the wing would be over-thick and slow. No existing plane has ever been made large enough to make it worthwhile. For it to make sense you need a payload upwards of 500 tons (equivalent to around 5,000+ passengers), six times the Airbus A380 or three An-225 cargo planes or two Stratolaunch Rocs. Oh, and the airports to fly it from.

Simple economics. Why spend billions and years designing a new plane from scratch - especially one that uses technology unproven in civilian applications (flying wing) - when you can spend millions and months buying passenger planes that use proven, tried-and-tested technology, and refit them for cargo needs?

Isn't this all already covered in Peter Kämpf's answer?

While all the other answers tackle quite a few practical problems that flying wing cargo planes would need to combat, there is also the problem that airplane operators tend to be very conservative when buying expensive aircraft. That's a major reason why commercial airplane design hasn't really changed in the last 50 years. Buying aircraft with a radical new design is risky. Better invest in proven technology that might less efficient rather than to risk loosing your whole investment if the new design turns out to be a failure.


text:
Why most military transport aircrafts use turboprop engines?
What is the reason behind that most (or many) military transport aircraft use turboprop engines?

It depends on the main purpose of the mission.
For example, for long haul flights, carring many pallets, they will use probably the turbo jets, because they have more endurance, and normally they will land on longer runways.

You may provide some contexte and links, and develop you answer. See this help page for more information.

Also, you can edit your previous answer to include more information instead of creating a new, nearly equally short, one.


text:
How is hypothetical max altitude of a helicopter calculated?
How do engineers calculate the hypothetical max altitude of a helicopter if the mass, altitude and length/curvature of blades are known?

There are different definitions for a ceiling (hovering in ground effect, hovering off ground effect, in forward flight). Helicopter Performance (FAA) and Helicopter Theory.

The helicopter reaches its ceiling if all installed power is used for propelling the helicopter forwards: there is no more extra power available to climb.

The figure from this answer shows the required power as a function of airspeed: it can be seen that there is an optimal true airspeed where required power has a minimum. It is at this airspeed that the helicopter reaches its OGE ceiling.
So it will be necessary to compute the four types of power at speed and height:

Propulsive & induced power, depending on rotor blade length, tip speed, blade chord and twist, solidity ratio.
Profile, depending on blade profile shape.
Parasitic, depending on fuselage shape, rotor mast shape etc.
Tail rotor, which is again propulsive & blade power of the tail rotor.

From the Source:

 It shows how to calculate theoretical maximum altitude a Helicopter can achieve

This answer really doesn't respond to the question of "how do we calculate the max altitude a helicopter can achieve". Of the four images included here, only the first contains relevant information, and only states the definition of the absolute ceiling, which is "the altitude at which max power equals required power". The referenced book itself also does not present the method to compute the maximum altitude.


text:
Why is it necessary to set the parking brake on for the walkaround on an A320?
Our Airbus A320 FCOM prescribes that the PM sets the PARK BRK ON before the walkaround.
Simple question: why?

There is something called a brake wear indicator on the bogie of the A320. To get an accurate reading from this, the parking brake needs to be on. It's not because if the brakes are off the plane will roll off, cause chocks are there.

In my experience, the break wear indicators - most of the time - are visible regardless of the position of the parking brake. Are they actually moving when the parking brake is applied?

The indicator pins are probably still visible with the parking brake released, but to get an accurate reading, they should be checked with the brakes set. The calibration is that the pin is flush with the rest of the assembly with brakes set when the pads are at their wear limit. With brakes released, there is movement and the top of the pin would no longer be flush, so the reading would be inaccurate.

And here I was envisioning the pilot, copilot and ground crew chasing the rolling A320 across the tarmac because Bob forgot to set the parking brake. Takes all the fun out of it! :)


text:
Do aircraft use their lights to 'say hello' to each other?
On the KLM website, I found this strange quote:

If planes pass each other at night they sometimes use light signals to say hello.

Is it true and common? How it is done (which lights/patterns)?

I wouldn't say that it would be common as if two aircraft are close enough to receive light signals, they would be in very close proximity of each other.. therefore posing a threat

I don't usually cruise with my landing and taxi lights on, but if I see another plane nearby around dusk (when my nav and beacon may not be bright enough to stand out against a bright sky, but the airplane fuselage may not be visible in the diminishing ambient light), I will turn on the rest of my lights to try to be more visible.   I'm not saying "hello", I'm just trying to clarify my presence.

You'd be surprised how far you can see in a clear sky @FallenUser, also keep in mind that 1000ft vertically is considered adequate separation under Reduced Vertical Separation Minimum regulations, which is plenty close.

@GdD Yes indeed I would and I would love to see how

@Glacomo Catenazzi: Did you mean the part in 'This is your captain speaking' where it says: 'If planes pass each other at night they sometimes use light signals to say hello.'?

@FallenUser a blinking light in the night should be visible for a really long distance in clear weather. I can easily see joggers with a single small head lamp a kilometer away at night.

@FallenUser: yes, that part.

Yes, it is true.  When two aircraft flying in opposite directions pass each other at night, it is quite common for one or both to briefly turn on their landings lights. This is done partially to "say hello", and also to increase situational awareness. I would guess it happens about 50% of the time.
Usually one aircraft will turn on their landing lights about 2-5 miles back and wait for the other to briefly flash their lights in response. It is not uncommon to get no response because the pilots on the other aircraft may have their heads down, and not see the other aircraft. Some pilots don't respond because they feel it is a useless practice, and can't be bothered.
When there is only 1,000' vertical separation of opposite direction, or crossing  traffic, some ATC jurisdictions might require the controller to inform the aircraft of each other's presence. In that case it is more likely that both aircraft will turn on their landing lights to be more visible to the other aircraft. There is no operational requirement to do this, but most pilots tend to go along with this practice.

Wouldn't the airlines see each other on TCAS anyway?  I have ADS-B In now, and  I appreciate being able to see other nearby traffic and their height above me or below me, and the direction they are heading.  I expect to see even more as Jan 1, 2020 comes around. (USA requirement for a lot, but not all, of the National Air System (NAS).)

"When there is only 1,000' vertical separation of opposite direction, or crossing traffic, ATC rules usually require the controller to inform the aircraft" not true

@J.Hougaard  I edited my answer to say “some ATC jurisdictions might require”.

In my experience, this is what I see too.  Some pilots do it, and some don't.


text:
Do any modern airliners have a non-flat floor in the passenger cabin?
Are there any non-flat (sloped or stepped) passenger cabins in modern commercial planes? E.g. the floor at the back of the cabin is a few centimeters higher than at the front?

@min: no, I mean floor of the cabin not flat, but e.g. going "up" in the tail. Recently I was in last rows (off from normal chairs) in an Airbus, and I had the impression that the last part of the cabin went up. Probably in my case was because of cabin form, but so I had such question:

Some Airbus aircraft have this characteristic, in the rear section of the cabin floor. You can see this in the following image. The rear passenger windows slope up following the floor slope.

A340 line drawings; image from wikimedia.org
This image of the upper cargo deck of A330 MRTT upper cargo deck (which would be the cabin floor in the passenger version) shows the upward slope.

A330MRTT cabin floor; image from thinkdefence.co.uk
Apparently, this is a common trait in A300/310/340/330's to allow more containerized cargo in the rear.

What I get from the drawings: A340-600 is really long!

The new honda jet has a stepped cabin floor but it is center/out not front/back. Note the recessed channel in the middle.

(source)
The Beech King Air (a fairly common charter plane) also has a single step as you move forward in the cabin

(source)

I wouldn't call either of these "airliners"

I admit the term airliner is a little ambiguous, but "this ain't it". Also, that King Air "step" is hardly a step. It is a bump about 6" wide that covers the wing spar carry through. Your picture just makes it look like a step up. The King Air does have the recessed channel in the center isle, however.

Oh, the monochrome! That HondaJet interior hurts my eyes!

@FreeMan I blame the lighting.

They must have just discovered beige.